The contact start-stop system (CSS system) is the leading type of start-stop system in hard disk devices using a flying type magnetic head. In the drive using "CSS system", a magnetic head for recording and reproduction contacts the surface of a disk at the time of the start and stop of the disk's rotation. The magnetic head slides on the surface of the disk while in contact with the surface of the disk. Because of this, the magnetic head and the disk are often damaged by abrasion, and if this becomes serious, head crash occurs.
Protective films composed of carbon, predominantly amorphous carbon, or silicon dioxide has been proposed on magnetic layers to prevent this problem.
The use of a liquid lubricant such as a perfluoroalkylpolyether coated on the protective film has also been proposed because the use of the protective film only is considered insufficient; see U.S. Pat. No. 3,778,308.
The distance between the magnetic layer and the magnetic head must be shortened to increase recording density. To achieve this, it is necessary to reduce both the thickness of the protective layer and the roughness of disk surface. For improved durability, a thicker lubricant layer is preferred. However, when this thickness is increased, it not only makes the disk unsuited for high-density recording, but also causes adsorption of the magnetic head to the surface of the disk. To prevent this, it has recently been proposed to reduce the thickness of the lubricant layer to only several tens of .ANG. which corresponds to a lubricant layer of only several molecules thick.
However, an interaction between the magnetic or protective layer formed by a thin film forming method such as dry-process plating (e.g., sputtering) or wet-process plating and the lubricant molecule is generally weak. Because of this, when the lubricant layer is only a few molecules thick, the lubricant is likely to be spun off or dissipated by centrifugal force when the disk is rotated or by negative pressure when the magnetic head passes by. These factors also decrease the durability of the lubricating function.
Methods in which a polar group is bonded to the terminal of a perfluoroalkylpolyether chain provide an increased interaction between the magnetic layer or the protective layer and the lubricant molecule have been proposed, thereby the durability of the lubricating function is improved without increasing the thickness of the lubricant layer as described in U.S. Pat. Nos. 4,267,238 and 4,268,556, and JP-B-60-10368 (the term "JP-B" as used herein means an "examined Japanese patent publication").
However, the present inventor has found that these procedures still insufficient.
Magnetic recording mediums are shocked and rubbed by the magnetic head and are subjected to very severe use conditions. It is unavoidable that the lubricating film coated on the surface of the protective layer is scattered or liable to be unevenly distributed on the outer periphery of the disk by a centrifugal force. To avoid these problems, it has been proposed to introduce a polar group into the terminal of the perfluoroalkylpolyether chain. However, it has been found that, when the polar group is introduced into the terminal, the interaction between the protective layer and the lubricant layer is still insufficient, and also the adsorption produced by the terminal polar group is apt to be detached. It has also been found that the adsorption produced by the terminal polar group is apt to be more easily detached with increasing in the length of the molecular chain when the molecular weight of the perfluoropolyoxyethylene group is increased.
It has now been found that, when a lubricant having a structure devoid of polar groups at the terminals of its molecule, but with polar groups on the intermediate carbon chain, both lubricating function and durability can be further improved.